Coated (or encapsulated) fertilizers are known to be very effective sources to provide controlled release of nutrients for the feeding of plants. The nutrients are released at controlled rates through the fertilizer's coating resulting in a sustained feeding of plants. As a result, one application of these so-called controlled release fertilizers can provide the necessary nutrients for a plant that would take multiple applications of soluble or non coated fertilizers.
These coated fertilizers may be classified into two major groups according to the fertilizer release mechanism. One type of coated fertilizer in wide use is sulfur coated fertilizer, such as disclosed in U.S. Pat. Nos. 4,042,366; 4,636,242 and 5,405,426. The release of nutrients from sulfur-coated fertilizers occurs by diffusion through imperfections in the sulfur coating and through coating breakdown. The major advantage of the sulfur coated fertilizers is their relatively low cost.
A second type of controlled release fertilizer utilizes solvent applied polymer coatings. The polymeric materials applied are either thermosetting resins or thermoplastics. Examples of solvent applied thermosetting resin coated fertilizers which are currently in use are disclosed in U.S. Pat. Nos. 3,223,518; 4,657,576 and 4,880,455; whereas examples of fertilizers having thermoplastic coatings are disclosed in U.S. Pat. No. 4,019,890. Another type of encapsulated fertilizer that exhibits good controlled release properties is latex coated granular fertilizers such as those disclosed in U.S. Pat. Nos. 4,549,897 and 5,186,732. Both solvent and latex applied polymer coated fertilizers offer important benefits over sulfur-coated products in regard to consistency of release rates. The majority of nutrient release is by diffusion through pores in the polymer coating, rather than release through coating imperfections.
Improvements to the process for producing a coated fertilizer have been investigated. U.S. Pat. No. 7,682,656 describes a process which aims to product a coated product having a low coating weight and a good slow release profile. The process comprises the steps of a) coating a substrate with a coating material to form a coated substrate; and b) stabilizing the coated substrate to form the coated product. The operating parameter of substrate-substrate contact and coated substrate-coated substrate contact differs between step a) and step b), such that in step b) the contact is minimized. In the example, the process is performed by using one rotating drum and applying different rotating speeds for the coating step and the stabilizing step.
There is a constant need in the industry for a more efficient process for producing a coated fertilizer.